1. Field of the Invention
This invention relates to an apparatus for setting the schedules of a resistance welding machine.
2. Description of the Prior Art
Resistance welding is widely employed in joining together the metal plates of assemblies such as automobile bodies and it is now possible to control the sequence of a series of resistance welding steps by microcomputer. The parameters that characterize a resistance welding operation, such as the duration of applied pressure and current and the magnitude of the applied current, are set in accordance with the particular welding sequence schedule.
Conventionally, methods of setting a resistance welding schedule fall into two main groups, namely methods that rely upon digital switches and those that employ keyboard switches.
The digital switch configuration is advantageous in that the switches themselves also serve as indicating means and enable the retention of set data even in the event of a power cutoff. On the other hand, the wiring connecting the digital switches with a control unit is of some complexity and the system is susceptible to failures such as poor electrical contact because the digital switches per se rely upon contact members. Still another disadvantage with the digital switch system is that a large number of these switches are necessary for a welding operation of the type in which one among a plurality of set values in a welding schedule is selected through use of a single control unit. The greater number of switches further complicates the wiring between the switches and the control unit and raises overall cost.
A prior-art example of a schedule setting method effected by keyboard switches is disclosed in the specification of Japanese Patent Application Laid-Open No. 58-176087. The art disclosed describes an arrangement in which such parameters as the duration of initial pressure application, the duration of current flow, i.e., conduction, and the magnitude of the current are set in accordance with a welding schedule by manually operating numeric keys for numerical values 0 through 9 as well as a variety of function keys. However, since one and the same display monitor is used to display all of the set values, the operator cannot readily tell which item of data is in the process of being set. The operator is also confronted with a large number of different types of keys. Such a configuration complicates the setting operation and invites operator input error.